The problem of preventing air embolism (obstruction of a blood vessel by an air bubble) during intravenous feeding has received mixed attention by the medical profession. The quantity of gas the circulatory system may sustain without hazard is difficult to estimate; recently, the figure of 40 to 50 cc (cubic centimeter) has been generally accepted as definitely hazardous to life. This amount is not small, so that at present, gravity-fed, intravenous feeding apparatus is almost adequate in impeding air intake should the IV bottle empty; provided the attendant nurse and/or patient is alert. Nevertheless, cases have been reported of deaths caused by air embolism through negligence.
Presently, the problem of detecting air bubbles in feeding tubes to prevent air embolism is receiving attention by engineers and doctors who are developing automated intravenous techniques: for treatment of hemodialysis patients, for precise application of fluids and medication in the operating room, and for replacing gravity-fed IV devices in the recovery ward. With these automatically-controlled active IV pumps, the problem of exceeding the 40-to-50 cc air tolerance level becomes more acute. The new techniques employ local or remote (computer) programming of fluid flow. The purpose of the bubble detector is then to stop the flow by mechanically clamping the feeding tube, to turn off the pump, to warn the attendant nurse, and to give a visual or audio indication of the precautionary state.
Possible methods of bubble detection in tubes are: (1) photoelectric, (2) electronic, (3) ultrasonic, and (4) other. The photoelectric technique has been utilized in a commercial bubble detector for dialysis tubing (by Vital Assists, Inc.) - but it is perhaps the most unreliable method: it is plagued by a low detection threshold level between, say, clear saline and air, and requires adjustments for fluids of different opacities.
The electronic method is based on the difference in electrical impedance between fluid and air. The real part of the impedance, that is the D.C. resistance, can only be measured by electrode contact within the fluid, and it obviously will depend on the fluid's ionic concentration. The purely imaginary part of the impedance, that is the capacitance, differs significantly (80.times.) between water and air, but sophisticated laboratory equipment is required to measure the slight capacitance changes caused by a small bubble (i.e., 1 mm in diameter). This parameter can be measured with plates outside the intravenous tube, which are connected to an AC-impedance bridge, for example. The bridge must be carefully balanced to a null when fluid is present in the vessel. This device has been reported in the literature by B. C. Taylor et al. "An Instrument for the Prevention of Air Embolism in Hemodialysis Patients," 25th American Conference of Engineers in Medicine and Biology Proceedings, 1972, P. 175. This method of detection is prone to errors due to inherent low threshold detection (i.e., detection of approximately a 1 picofarad capacitance change), and still requires complex analog hardware costing hundreds of dollars in a stand-alone unit. Also, frequent calibration is probably necessary using this technique.
The ultrasonic approach is based on the large acoustic impedance mismatch between liquid and air, and the relatively insignificant acoustic impedance difference between water-based fluids of different concentrations, opacities, and colors. Some researchers have used ultrasonics for bubble detection. For example, the detection of small bubbles in the blood vessels of divers with decompression sickness has been reported by G. J. Rubissow and R. S. Mackay, "Ultrasonic Imaging of Bubbles in Decompression Sickness," Ultrasonics, Vol. 9, No. 4 (1971), p. 225; R. Y. Nishi, "Ultrasonic Detection of Bubbles with Doppler Flow Transducers," Ultrasonics, Vol. 10, No. 4, p. 173 (1972); A. Evans and D. N. Walder, "Detection of Circulating Bubbles in the Intact Mammal," Ultrasonics, Vol. 8, No. 4, p. 216 (1970).
One prior art method using ultrasound for detecting bubbles in a small tube is to immerse it in a water tank lined with a sonic absorbent (i.e., anechoic chamber for ultrasound). The tube is then inspected with an "ultrasonoscope," a device used in non-destructive testing. It generates ultrasonic bursts, then detects backscattered echoes in a sonar fashion, displaying them on a cathode-ray tube (amplitude vs. time). The detection of large echoes from the area of the tube indicates the presence of air. To implement such a system requires elaborate, bulky equipment. Engineers working in the field have come to regard the ultrasonic approach as the most cumbersome and hence the least attractive of the modalities applicable to this problem.
One report in the literature, "Ultrasonic Detection of Gas Bubbles in Blood," by D. M. J. P. Manley, Ultrasonics, April, 1969, pertains to an ultrasonic through-transmission bubble detector for fluids in feeding tubes. the prototype device reported consists of a dual-section chamber, for intake and outlet of fluid, and a small orifice connecting the sections designed to control fluid flow. Ultrasonic transmitter and receiver crystals are coupled appropriately to opposing sides of the lower and upper chamber, respectively, so that ultrasonic energy to the receiver must flow through the orifice in the same direction as fluids flow.
The impracticality of the device is apparent when considering the elaborate and bulky equipment required, the poor performance achieved, and the fact that the device is invasive to the tubing, i.e., requiring liquid to actually flow through the detector chambers. Manley used an operating frequency of 43 KHZ, since at the higher frequencies, the effects of air bubbles on the strength of the received signal were not so prominent. At this low ultrasonic frequency, the wavelength is large (37 mm in water), requiring large transducers, large transmitter amplifier power, and a long path length for the orifice. The actual device as described used 2 inches thick, 2 inches diameter transducers as transmitter and receiver, a stable frequency oscillator and a power amplifier to drive the transmit transducer, and at the receiver a low-frequency tuned-amplifier, detector, and postamplifier. The dimensions of the chamber and orifice are not given, but they are shown as large compared to an IV feeding tube (typically around 5 mm). Manley reports getting a 9-10 db signal reduction by the simultaneous presence of 30 bubbles, around 1 mm each, in the path of the transducers. Since the bubbles are many times smaller than the ultrasonic wavelength, the mechanism of attenuation is acoustic absorption, probably due to thermal compression at the bubble sites. Because this kind of attenuation is small, only the effect of many bubbles and long path length (at least 3 or 4 wavelengths in water) are required. other deficiencies include: (1) due to the low operating frequency external vibrations of the chamber produce marked effects [.e., false alarms]. Isolation is needed for experiments, as anti-vibration mounts are essential for these chambers. (2) The relatively high energy at low frequencies is known to be damaging to blood cells, especially those streaming at the transmitter transducer.
other approaches may be based on the differential thermal conductivity properties between air and water, although there does not appear to be any work reported in the literature using this technique.